Add cosserat curve

[isteinbrecher]

Add a new submodule cosserat_curve that an be used to describe general Cosserat curves with minimal twist.

[isteinbrecher]

@knarfnitram can you have a look, this should be the full functionality of the CosseratCurve module. I improved some internal details and the test suite should now cover most of the functionality.

@davidrudlstorfer this module is tested with pytest, I simply copied the relevant utility functions in tests/utils.py to also work with pytest.

[davidrudlstorfer]

Very nice that you started with the pytest framework @isteinbrecher!

The changes look really nice! I have a few remarks (unused variables and commented code) and a few small discussions regarding the change to pytest.

You do not need to apply pytest suggestions in this PR - I am happy to adjust things once more tests move to pytest.

P.S.: I also did not look into the cosserat curve and simply reviewed the testing

[knarfnitram]

Thanks for adding the warping by Cosserat curve to MeshPy @isteinbrecher .

I have only a few remarks.

[davidrudlstorfer]

Looks good! I've added the points regarding pytest to the issue #119 and I'll work on them soon